1. Technical Field
The preent invention relates generally to a suspension control system for an automotive vehicle. More particularly, the invention is directed to a damping force control system for a variable damping force shock absorber which is operable to modify damping characteristics based on vertical speed of a sprung portion of a vehicle and relative displacement speed between the sprung and unsprung portions of the vehicle.
2. Background Art
Japanese Patent First Publication No. 3-104726 discloses a damping force control system using the sky-hook theory which is designed to modify a damping force of a shock absorber between maximum and minimum levels in a stepwise fashion based on a vertical speed DX of a sprung portion and a relative displacement speed DY between the sprung and unsprung portions of a vehicle.
The above prior art damping force control system offers the advantage in that a high damping force is established against vibrations of relatively lower frequencies due to gradual changes in road elevation for optimizing vehicle handling, road holding ability, and riding comfort. A drawback is, however, encountered in that a damping force higher than that required (i.e., the maximum level) is provided against vibrations including relatively high frequency components which are often experienced in a city road wherein the vertical speed DX and the relative displacement speed DY represent a relatively small value respectively. This gives an unpleasant feeling to vehicle occupants which is caused by the mechanical shock due to a damping force switching operation of the shock absorber.
For improving firm riding comfort, one method has been proposed which slows damping force switching speed only when a damping force control valve is switched from a closed position to an open position to reduce the damping force switching shock. This method is effective in reducing the damping force switching shock, however, it has suffered from a drawback in that a suitable damping force cannot be provided over a wide range from high to low frequency vibrations.